1. Field of the Invention
The present invention relates to the field of azimuth determination, with applications e.g. in the field of pedestrian or vehicle autonomous navigation. More particularly, the invention provides a method, apparatus and computer program which optimizes the accuracy obtained from sources of raw azimuth data.
2. Prior Art
An accurate and reliable determination of azimuth is important notably in navigation, surveying and civil engineering. Known techniques rely on external signal sources to acquire azimuth data, such as in GPS navigation systems, or in systems which receive signals from land-based radio beacons. However, the reliance on being able to receive the necessary external signals exclude these techniques where the observer is susceptible of being in poor or impossible reception conditions, such as indoors, in densely urbanized or forested environments, or under a low cloud ceiling in the case of a GPS receiver.
It is then necessary to use autonomous, or dead reckoning, azimuth determination techniques. Typically these involve the use of a magnetic compass or a gyroscope.
A magnetic compass indicates the direction of alignment of the Earth's magnetic flux lines, which join at the magnetic North. Modern magnetic compasses use semiconductor magnetic sensors based on the Hall effect, and can produce an output in electronic form. A magnetic compass responds simply to ambient magnetic field lines, which may have other components than the Earth's magnetic field. These spurious components can be magnetic fields from nearby man-made sources, such as transformer stations, transmission lines, electric motors, etc., or from natural conditions, such as iron ores, magnetic storms, etc. They produce errors in the compass reading which cannot be easily detected or corrected, with an attendant loss of azimuth accuracy that is incompatible with many applications. Also, the Earth's magnetic field lines can themselves be distorted, e.g. by metallic structures, features of the local geophysical environment, etc.
A gyroscope uses the gyroscopic or rotational effect of a rotating or vibrating mass to maintain a pre-programmed reference azimuth. Small navigation systems typically use a portable gyroscope in the form of a combined mechanical and electronic unit comprising two or three orthogonal reference azimuth axes from respective gyroscopes. The absolute reference azimuth is maintained in alignment during the gyroscope's displacement, and is unaffected by spurious magnetic fields. However, it is subject to a systematic error as a function of time, referred to as drift, which calls for frequent reprogramming cycles of the reference azimuth, or the gyroscope bias.